The present invention relates to high performance thermal imaging sensors. The proposed device technology has the potential to reach background limited performance due to a novel non-contacting measurement approach and low device noise. This invention is based on the steep, reversible change in AC magnetic susceptibility that occurs at the magnetic Curie temperature. The device is a bolometer that uses a microbridge structure to isolate the pixel. It is similar in many aspects to high Tc superconducting (HTS) transition edge bolometers and ferroelectric detectors that operate at the transition edge. However, there are several key features that distinguish it from those technologies that are described below.
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The principal distinguishing feature between existing thermal imaging sensors and the present invention is that electrically, the device is an inductor rather than a resistor or capacitor. Since the invention uses an inductive approach to probe the temperature sensitive material, there are no connecting leads required to the microbridge element. The temperature sensitive material is suspended above, and substantially completely isolated from the underlying substrate and interrogation inductor.
Another unique feature of the invention is that the device will have an inherently high sensitivity due to the steep paramagnetic-ferromagnetic phase transition. This reversible change in the AC susceptibility (permeability) of magnetic alloys near the Curie temperature is called the Hopkinson effect, and has been demonstrated for several magnetic alloys. In addition, the resistance of the device is quite small, resulting in a reduced Johnson noise compared with standard VOx based devices.
The approach is also unique in that it allows for considerable flexibility in the pixel design, device operating temperature, and device fabrication for the following reasons:
The non-contact measurement geometry;
The wide range of Curie temperatures (covering the range. specified in the BAA from 77 K to 300 K) obtainable simply by varying the alloy composition (see FIG. 6); and
The ease of processing accorded when using simple binary metal alloys as the temperature sensing material. (Note: The same non-contacting measurement approach can be applied to HTS transition edge devices. However, HTS materials are much more complicated in comparison to the simple binary alloys proposed.)
The final unique feature is that the proposed approach brings an extensive technology base in magnetic recording head and measurement technologies to the development of microbolometer devices.